89V51 MICROCONTROLLER & GSM900A DOES NOT COMMUNICATE EVERYTIME

Hello..
I am doing one project in which msg from GSM is read and it is Displayed to LCD.
but microcontroller 89v51 and GSM900A does not communicate every time..
sometimes it successfully happen that AT command from microcontroller is send to GSM and message from any no. to GSM is Displayed to LCD..
bt it's not happening everytime..
i am not getting the solution, Please help..
I have connect 12V,2A supply to GSM..
Microcontroller Rx pin connected to Tx of GSM..
Microcontroller Tx pin Connected to Rx of GSM..
and Ground to Ground..
Please help me.
what may be the problem?
and how should i fix that.

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